The Samsung Galaxy S II is 0.33" (8.49mm), which is less than even the Sony Ericsson Xperia arc, which is 0.34" (8.7mm) in its thinnest part. A big part of it, besides the proven engineering prowess of Samsung Electronics, is no doubt the Super AMOLED Plus screen. It needs much less layers than an LCD-based display, thus leaving room for the rest of the components in a thinner chassis.
The phone is actually slimmer than its tiny mobile camera sensor, and the 8MP shooter slightly protrudes from the back. Samsung's trademark "hump" at the back bottom is the only place where the Galaxy S II equals in thickness with the iPhone 4, but, of course, we shouldn't forget Apple's creation is made of glass, not plastic. Have a look at the quick comparison video below.